Microencapsulated electrophoretic display with integrated driver

ABSTRACT

A mounted display assembly comprises a flexible substrate that supports both display elements and control circuits. The display assembly generally comprises: an electrical connection formed on the flexible substrate, the electrical connection having first and second contact pads; a display element in electrical communication with the first contact pad; and a control circuit mounted on the flexible substrate and in electrical communication with the second contact pad. In a preferred embodiment, the display element comprises a microencapsulated electrophoretic display medium. In another preferred embodiment, printing processes are employed in manufacturing methods for the display assembly.

TECHNICAL FIELD

[0001] The invention relates generally to panel-type display devices andmore particularly to flexible electrophoretic displays.

BACKGROUND OF THE INVENTION

[0002] Panel-type electronic display devices typically require a rigidcircuit board mounted control circuit. For example, the liquid crystaldisplays found in laptop computers typically have several integratedcircuits mounted on circuit boards, the circuit boards arranged aroundthe liquid crystal portion of the panel. As panels of increasing sizeand resolution are developed, panels tend to require larger and heaviercircuit boards in the manufacture of the display.

[0003] Such printed circuit boards are expensive to manufacture andpresent the additional cost and complexity of physical and electricalinterfacing with other display components. The added manufacturing stepsrequired to connect the electrical conductors on the display mediumportion of a display, e.g. the liquid crystal portion, with theelectrical conductors on a circuit board also lead to yield loss.

[0004] It would be desirable, for many applications, to have thin,flexible displays, though liquid crystal media are not well suited touse with flexible substrates. Combined use of flexible substrates andlower cost conductor printing methods holds the potential of lower costdisplays for a variety of uses, such as: rolled displays; affordablelarge area displays; displays incorporated into fabrics; and as a papersubstitute. Unfortunately, the cost of circuit boards and the mating ofcircuit boards to substrates are two impediments to realization of theadvantages of flexible displays.

SUMMARY OF THE INVENTION

[0005] In a broad sense, the invention provides simpler, lower costmanufacturing methods and realization of the advantages of flexibledisplays through better use of flexible substrates. In one aspect, theinvention provides a lower-cost, more flexible, and more useful displaydevice through an electrophoretic display assembly and method ofmanufacturing the electrophoretic display assembly. In one embodiment,the display assembly comprises: a flexible substrate; an electricalconnection formed on the flexible substrate; the electrical connectionhaving first and second contact pads; an electrophoretic display elementin electrical communication with the first contact pad; and a controlcircuit mounted on the flexible substrate and in electricalcommunication with the second contact pad.

[0006] A method of manufacturing the electrophoretic display assembly,in one embodiment, comprises: formation of electrical connections,including contact pads, on a flexible substrate; mounting a controlcircuit on the flexible substrate by bonding control circuit leads tothe contact pads; and forming one or more electrophoretic displayelements on the flexible substrate, where the control circuit drives thedisplay elements.

[0007] In the case of prior art display assemblies with display elementson rigid or flexible substrates, control circuitry is typically mountedon rigid circuit boards. The substrate portion of the display assemblyand the control circuitry portions must then be physically joined. Thisapproach has cost and reliability disadvantages.

[0008] In contrast, the present invention, in one aspect, providesco-location of display elements and control circuitry on a shared,flexible substrate. This permits manufacturing of a flexible paneldisplay. In one embodiment, use of an electrophoretic display medium, inparticular an encapsulated electrophoretic display medium, leads to aflexible display that can be substantially flexed without substantialdetrimental impact on the optical performance of the display medium.

[0009] Use of an encapsulated electrophoretic display medium furtherpermits use of lower cost printing methods for deposition of the displaymedium. In a preferred embodiment, the display assembly provides forprinting of electrical connections between display elements and controlcircuitry in a single printing step.

[0010] In one aspect, the invention eliminates the manufacturing stepsthat would be entailed in electrically and physically joining separatedisplay medium substrate and control circuit substrate portions of adisplay. In the preferred embodiment of a common flexible substrateshared by the display medium and the control circuit, joining steps areeliminated, cost and yield are improved, and an overall more flexiblestructure is obtained.

[0011] The invention permits advantageous use of an electrophoreticdisplay medium. An electrophoretic display has attributes of goodbrightness and contrast, wide viewing angles, state bistability, and lowpower consumption when compared with liquid crystal displays. Inparticular, use of an encapsulated (or “microencapsulated”)electrophoretic display medium provides advantages, such as the abilityto print or coat the display medium on a wide variety of flexible orrigid substrates. Further, because the display medium can be printed(using a variety of methods), the display itself can be made lessexpensively.

[0012] A microencapsulated electrophoretic display medium is well suitedto flexible display applications, since it can tolerate a high degree offlexing without substantial detrimental impact on its opticalperformance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The invention, in accordance with preferred and exemplaryembodiments, together with further advantages thereof, is moreparticularly described in the following detailed description, taken inconjunction with the accompanying drawings.

[0014] In the drawings, like reference characters generally refer to thesame parts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating principles of the invention.

[0015]FIG. 1 shows a diagrammatic planar view of an embodiment of adisplay assembly.

[0016]FIG. 2a shows a planar view of an alternative embodiment of thedisplay assembly of FIG. 1.

[0017]FIG. 2b shows a cross-sectional view that generally corresponds tothe display of FIG. 2a.

[0018]FIG. 3a shows a planar view of an alternative embodiment of thedisplay assembly of FIG. 1.

[0019]FIG. 3b shows a cross-sectional view that generally corresponds tothe display of FIG. 3a.

[0020]FIG. 4a shows a planar bottom view of an embodiment of afour-character display assembly.

[0021]FIG. 4b shows a top view of the four-character display assembly ofFIG. 4a.

[0022]FIG. 4c shows the configuration of display elements of one of thecharacters of the assembly of FIG. 4b.

[0023]FIG. 5 shows a flow chart of a typical display assemblymanufacturing process.

[0024]FIG. 6 shows a flow chart of an embodiment of a manufacturingprocess for a flexible display assembly.

DETAILED DESCRIPTION OF THE INVENTION

[0025] In broad overview, the invention entails support of anelectrophoretic display medium, control circuits, and electricalconductors on a common flexible substrate. This leads to manufacturingwith lower cost of processing steps, and higher product yield. Theresulting flexible display assembly has many advantageous uses, forexample, in production of large area displays or display devices thatcan be flexed or rolled.

[0026] I. An Electrophoretic Display Assembly

[0027]FIG. 1, in broad overview, depicts a schematic representation ofan embodiment of an electrophoretic display assembly 100. The displayassembly 100 comprises: electrophoretic display elements 110, eachelement 110 corresponding to a single pixel of the display assembly 100;a control circuit 130, the control circuit 130 in electricalcommunication with the display elements 110 via drive signal electricalconnections 120, and in electrical communication with other components(not shown) either present on or off of a flexible substrate 140 viaother electrical connections 150, where the display elements 110, thecontrol circuit 130, and the drive signal electrical connections 120 aresupported by the flexible substrate 140. The terms “control circuit” and“control circuitry” are here used interchangeably and can comprisesingle or multiple components.

[0028] For simplicity, nine display elements 110 are shown in FIG. 1.Generally, however, the display assembly 100 would include a largernumber of display elements 110. Further, a variety of shapes can beemployed for the display elements 110 to provide, for example, a morepleasing appearance in alphanumeric data presented by the displayassembly 100.

[0029] The control circuit 130 is generally comprised of one or moreintegrated circuits (the terms “IC” or “chip” are here usedinterchangeably with “integrated circuit”), such as driver chips,interface chips and other control chips. In one embodiment, the controlcircuit 130 comprises one or more driver chips, the driver chipssupplying drive voltages to the display elements 110. In an alternativeof this embodiment, interface chips, either supported on or off theflexible substrate 140 can mediate electrical communication between oneor more driver chips mounted on the flexible substrate 140 and otherIC's mounted either on or off the flexible substrate 140.

[0030] In further detail, referring to FIG. 2a, an embodiment of adisplay assembly 140 with four display elements 110 in electricalcommunication with a single driver chip 131 is shown. The drive signalelectrical connections 120 comprise four individual electricalconnections 123, one for each of the four display elements 110 depictedin FIG. 2.

[0031] The individual electrical connections 123 are lines of conductivematerial, the conductive material being deposited via a number ofpossible processes. In a preferred embodiment, the conductive materialis deposited by printing methods, employing, for example, electricallyconductive ink. This provides for relatively low cost processing.Further, electrical connections 123 can be formed in a single step,further reducing processing cost and enhancing manufacturing yield.

[0032] The display 100 can employ various materials. The flexiblesubstrate 140 can comprise a polyester sheet with electrical connections123 formed of copper by conventional patterning techniques.Alternatively, the electrical connections 123 can be printed with silverink or carbon ink. The electrical connections can be coated by printingwith a dielectric, for example a polymer. Vias through the dielectriccan provide for electrical contact to a display element 110. Eachelectrical connection 123 is in communication with a first contact pad121 and a second contact pad 122. Further, each first contact pad 121 isin electrical communication with one of the display elements 110 whileeach of the second contact pads 122 is in electrical communication withthe driver chip 131. The driver chip 131 is in electrical communicationwith other contact pads 151 to provide for electrical communication withother IC's (not shown) of the control circuit 130.

[0033] Referring to FIG. 2b, a cross-section view corresponding ingeneral principles to the embodiment of FIG. 2a is shown. In thisembodiment, the display element 110 is comprised of: a pixel electrode111, an electrophoretic display medium 113; and a second electrode 112.In this embodiment, each display element 110 has its own pixel electrode111 while the second electrode 112 can be shared by more than onedisplay element 110. That is, a common second electrode 112 can extendacross multiple display elements 110, and preferably extends across allthe display elements 110.

[0034] The first contact pad 121 can contact the pixel electrode 111along the side of the pixel electrode 111, as indicated in FIG. 2a.Alternatively, the first contact pad 121 can contact the pixel electrode111 at any location on a surface of the pixel electrode 111, thoughpreferably on a surface opposite to the display medium 113.

[0035] In the embodiment of FIG. 2b, the driver chip 131 makeselectrical contact with the second contact pads 122 and other contactpads 151 through leads 132 and a bonding material 124. The bondingmaterial comprises any material that is suitable for physically securingelectrical communication between a lead 132 and a contact pad 124, forexample an anisotropic conductive film (ACF), a conductive epoxy (suchas silver-filled epoxy), an electrically conductive thermoset, silverpaint, an electrically conductive ink, or an electrically conductivepaint.

[0036] Alternatively, the driver chip leads 132 can be physically andelectrically fixed to the contact pads 122, 151 via compression bonding.In a further alternative, the driver chip 131 can be mounted on theflexible substrate 140 through a socket device (not shown), where thesocket device is supported by the flexible substrate 140 and inelectrical communication with the contact pads 122 and 151. For example,the driver chip 131 or other control circuit 130 chips can be removablymounted in a control circuit carrier.

[0037] Referring to FIG. 3a, an alternative embodiment of theelectrophoretic display assembly 100 has individual electricalconnections 123 that are electrically isolated by an insulating layer(shown only in FIG. 3b) from the pixel electrodes 111. Referring to FIG.3b, a cross-section view of an embodiment that corresponds to thegeneral principles of the embodiment of FIG. 3a is shown. An insulatinglayer 114 provides electrical isolation between the electricalconnection 123 and the pixel electrode 111. In general, the insulatinglayer 114 provides electrical isolation between the pixel electrode 111and the portion of the electrical connections 120 that lie between thepixel electrode 111 and the flexible substrate 140.

[0038] In the embodiments of FIGS. 3a and 3 b, a conductive via 125provides electrical communication between the contact pad 121 and thepixel electrode 111. The via 125 provides a conductive pathway throughthe insulating layer 114. The via 125 and the insulating layer 114 canbe formed by a number of processes, for example by printing ofdielectric and conductive materials.

[0039] In an alternative embodiment of the display assembly 100, theorder of deposition of materials 120, 114, 111, 113, 112 on the flexiblesubstrate 140 is inverted. In an example of this embodiment, theflexible substrate 140, such as a 4 mil thick polyester sheet, isdeposited, through a printing process, or by vacuum deposition, atransparent conductive coating for the electrode 112. The electrode 112can comprise a conductive polymer. A microencapsulated electrophoreticdisplay medium 113 is then printed upon the conductive coating, followedby printing of a patterned conducting layer comprising, for example,graphite or silver. The patterned conducting layer comprises the pixelelectrode 111. Intermediate to the display medium 113 and the patternedconducting layer, a insulating layer 114 comprised of a printeddielectric can be deposited as well as deposit of vias 125 by printing.

[0040] In a further alternative embodiment, the electrode 112 and amicroencapsulated electrophoretic display medium 113 are deposited on asecond flexible substrate (not shown) followed by lamination of thesecond flexible substrate to the flexible substrate 140. After thelamination process, the electrophoretic display medium 113 is adjacentto the pixel electrode 111.

[0041] In a preferred embodiment, the display medium 113 comprises amicroencapsulated electrophoretic medium. Microcapsules have, forexample, a diameter in a range of approximately 20 to 500 micrometers.The optical performance of such a medium is substantially unaffected bycurvatures with a radius of ten times or less the typical radius ofmicrocapsules in the medium. For example, for microcapsules with aradius of 150 micrometers, the medium can sustain a bend with a radiusof 1.5 millimeters or less.

[0042] The pixel electrodes 111 address and are in proximity to theelectrophoretic display medium 113. The display medium 113 haselectrically-responsive optical properties. By selectively altering theoptical properties of the display elements 110 using the pixelelectrodes 111, images or text can be displayed. As used herein, theterm “proximity” refers to a distance through which a voltage may beapplied to the display element thereby to alter its optical propertiesin a localized manner. As noted in embodiments described above, thepixel electrodes 111 are adjacent to the display medium 113 and can bein contact with the display medium 113.

[0043] The pixel electrodes 111 can be used to address a variety ofdifferent types of display elements 110, including, but not limited to,those with non-light emissive display media, for example, liquidcrystals, and bichromal spheres.

[0044] Now referring to FIGS. 4a-4 c, an embodiment of a four-characterdisplay assembly 400 is schematically depicted. FIG. 4a shows a bottomview of the display assembly 400. FIG. 4b shows a top view of thedisplay assembly 400.

[0045] The display assembly 400 includes four character display units401, each capable of displaying, for example, a letter or number. Eachdisplay unit 401 includes sixty three display elements 101 a. FIG. 4cshows the configuration of the sixty three display elements 101 a of oneof the characters of the assembly 400, from a top view perspective. Thisconfiguration of display elements 101 a is well suited for displayingalphanumeric characters.

[0046] Drive signal electrical connections 120 a electrically connectdriver chips 131 a (one for each display unit 401) to the displayelements 101 a. Hence, each driver chip 131 a is connected by sixtythree individual electrical connections to its associated characterdisplay unit 401.

[0047] The display assembly 400 includes a flexible substrate 140 a andother electrical connections 150 a to connect the driver chips 131 a toother components. For example, a series of display assemblies 400 can becombined to create a display with more than four characters in a row ormore than one row of characters.

[0048] II. Method Of Manufacturing An Electrophoretic Display Assembly

[0049] As discussed above, the various embodiments of the displayassembly 100 enable lower cost, higher yield manufacturing processes aswell as flexible display devices. Advantages of the invention areillustrated in the following discussion of manufacturing methods.

[0050]FIG. 5 shows a flowchart of an example of a prior artmanufacturing process for a display assembly. Firstly, electricalconnections are formed on a flexible substrate (Step 510 a).Independently, electrical connections, including contact pads, must beformed on circuit boards (Step 510 b). Display elements are thenprovided on the flexible substrate (Step 520) while, againindependently, control circuit leads are bonded to the contact pads onthe circuit boards (Step 530). The flexible substrate and the circuitboards must then be physically mounted to each other either directly orindirectly (Step 540) and electrical contact must be made between theelectrical connections on the flexible substrate and the electricalconnections on the circuit boards (Step 550).

[0051] In the prior art approach, an additional row of contact pads aretypically formed along one edge of the flexible substrate. This row ofcontact pads is then used for electrically mating the flexible substrateportion of the display assembly with the circuit boards.

[0052] The circuit boards employed in the prior art process aretypically heavy, expensive, and rigid. Use of circuit boards thus leadsto an inherently less flexible display assembly. Connecting the circuitboards with the substrate leads to cost and yield loss through addedmanufacturing steps. In particular, the necessity of forming electricalcontact between the electrical connections on the various componentparts of the display leads to added cost, time in manufacture and yieldloss.

[0053] In contrast, referring to FIG. 6, an embodiment of amanufacturing process 600 for a display assembly as contemplated by thepresent invention provides several advantages over the process describedabove. Firstly, electrical connections, including contact pads, areformed on a flexible substrate (Step 610). Then electrophoretic displayelements are provided on the substrate (Step 620). Lastly, controlcircuits leads are bonded to the contact pads (Step 630). In thisembodiment, the lead-contact pad bonding provides both electricalcommunication and physical support for the control circuits on thesubstrate.

[0054] This embodiment eliminates various disadvantages in prior artmanufacturing methods. Electrical connections that serve displayelements and control circuits can be formed in a single, cost and yieldimproving step. Size, weight, and overall flexibility are improved byelimination of rigid circuit boards. In addition to cost savings, thisembodiment permits realization of advantages of flexibility offered byuse of a flexible substrate. For example, use of a display assemblymanufactured in this manner permits fabrication of a flexible displaydevice that can be rolled for storage purposes.

[0055] In a preferred embodiment, an encapsulated electrophoreticdisplay assembly is manufactured with use of printing or coating stepson a wide variety of flexible substrates. As used herein, the term“printing” includes all forms of printing and coating, including, butnot limited to, pre-metered coatings such as patch die coating, slot orextrusion coating, slide or cascade coating, and curtain coating: rollcoating such as knife over roll coating, forward and reverse rollcoating, gravure coating, dip coating, spray coating, meniscus coating,spin coating, brush coating, air knife coating, silk screen printingprocesses, electrostatic printing processes, thermal printing processes,and other similar techniques. Thus, the resulting display can beflexible. Further, because the display medium 113 can be printed (usinga variety of methods), the display itself can be made inexpensively.

[0056] In a preferred embodiment, a microencapsulated electrophoreticdisplay medium 113 comprising, in part, a flexible binder material, isemployed. Such a display medium 113 is amenable to significant flexing.Flexing of the display medium 113 does not affect the optical appearanceof the medium. That is, the electrophoretic particles remain in the sameposition within the microcapsules without regard to the overallorientation or curvature of the binder or display.

[0057] Further, printing methods can be used to form the electricalconnections and other conductive portions of a display. A rear conductor(“rear” referring to a side of a display that is opposite to that viewedby a user) can be ether opaque or transparent. This allows the use of avariety of printed rear conductors, including graphite inks, silverinks, or conductive polymers.

[0058] The front conductor (“front” referring to a side of a displaythat is viewed by a user) must be transparent, but need not haveexcellent conductivity. Even materials with relatively poorconductivity, though amenable to printing, can be employed, for exampleconductive colloidal suspensions and conductive polymers such as arecommonly used in anti-static applications.

[0059] A microencapsulated electrophoretic medium, unlike a liquidcrystal medium, is amendable to use with a wide number of intrinsicallyconductive polymer systems, including derivatives of polyaniline,polypyrrole, polythiophene, and polyphenylenevinylene.

[0060] In short, the present invention permits a more advantageous useof cost savings and mechanical flexibility allowed by use of printingmethods for formation of conducting materials in a display assembly.

[0061] The following describes in detail various embodiments ofmaterials with applications to the electrophoretic display medium 113.

[0062] III. Materials for Use in Electrophoretic Displays

[0063] Useful materials for constructing the above-describedencapsulated electrophoretic displays are discussed in detail below.Many of these materials will be known to those skilled in the art ofconstructing conventional electrophoretic displays, or those skilled inthe art of microencapsulation. The combination of these materials andprocesses, along with the other necessary components found in anencapsulated electrophoretic display, comprise the invention describedherein.

[0064] A. Particles

[0065] There is much flexibility in the choice of particles for use inelectrophoretic displays, as described above. For purposes of thisinvention, a particle is any component that is charged or capable ofacquiring a charge (i e., has or is capable of acquiring electrophoreticmobility), and, in some cases, this mobility may be zero or close tozero (i.e., the particles will not move). The particles may be neatpigments, dyed (laked) pigments or pigment/polymer composites, or anyother component that is charged or capable of acquiring a charge.Typical considerations for the electrophoretic particle are its opticalproperties, electrical properties, and surface chemistry. The particlesmay be organic or inorganic compounds, and they may either absorb lightor scatter light. The particles for use in the invention may furtherinclude scattering pigments, absorbing pigments and luminescentparticles. The particles may be retroreflective, such as corner cubes,or they may be electroluminescent, such as zinc sulfide particles, whichemit light when excited by an AC field, or they may be photoluminescent.Finally, the particles may be surface treated so as to improve chargingor interaction with a charging agent, or to improve dispersibility.

[0066] A preferred particle for use in electrophoretic displays of theinvention is Titania. The titania particles may be coated with a metaloxide, such as aluminum oxide or silicon oxide, for example. The titaniaparticles may have one, two, or more layers of metal-oxide coating. Forexample, a titania particle for use in electrophoretic displays of theinvention may have a coating of aluminum oxide and a coating of siliconoxide. The coatings may be added to the particle in any order.

[0067] The electrophoretic particle is usually a pigment, a polymer, alaked pigment, or some combination of the above. A neat pigment can beany pigment, and, usually for a light colored particle, pigments suchas, for example, rutile (titania), anatase (titania), barium sulfate,kaolin, or zinc oxide are useful. Some typical particles have highrefractive indices, high scattering coefficients, and low absorptioncoefficients. Other particles are absorptive, such as carbon black orcolored pigments used in paints and inks. The pigment should also beinsoluble in the suspending fluid. Yellow pigments such as diarylideyellow, hansa yellow, and benzidin yellow have also found use in similardisplays. Any other reflective material can be employed for a lightcolored particle, including non-pigment materials, such as metallicparticles.

[0068] Useful neat pigments include, but are not limited to, PbCrO₄,Cyan blue GT 55-3295 (American Cyanamid Company, Wayne, N.J.), CibacronBlack BG (Ciba Company, Inc., Newport, Del.), Cibacron Turquoise Blue G(Ciba), Cibalon Black BGL (Ciba), Orasol Black BRG (Ciba), Orasol BlackRBL (Ciba), Acetamine Blac, CBS (E. I. du Pont de Nemours and Company,Inc., Wilmington, Del.), Crocein Scarlet N Ex (du Pont) (27290), FiberBlack VF (DuPont) (30235), Luxol Fast Black L (DuPont) (Solv. Black 17),Nirosine Base No. 424 (DuPont) (50415 B), Oil Black BG (DuPont) (Solv.Black 16), Rotalin Black RM (DuPont), Sevron Brilliant Red 3 B (DuPont);Basic Black DSC (Dye Specialties, Inc.), Hectolene Black (DyeSpecialties, Inc.), Azosol Brilliant Blue B (GAF, Dyestuff and ChemicalDivision, Wayne, N.J.) (Solv. Blue 9), Azosol Brilliant Green BA (GAF)(Solv. Green 2), Azosol Fast Brilliant Red B (GAF), Azosol Fast OrangeRA Conc. (GAF) (Solv. Orange 20), Azosol Fast Yellow GRA Conc. (GAF)(13900 A), Basic Black KMPA (GAF), Benzofix Black CW-CF (GAF) (35435),Cellitazol BNFV Ex Soluble CF (GAF) (Disp. Black 9), Celliton Fast BlueAF Ex Conc (GAF) (Disp. Blue 9), Cyper Black IA (GAF) (Basic Blk. 3),Diamine Black CAP Ex Conc (GAF) (30235), Diamond Black EAN Hi Con. CF(GAF) (15710), Diamond Black PBBA Ex (GAF) (16505); Direct Deep Black EAEx CF (GAF) (30235), Hansa Yellow G (GAF) (11680); Indanthrene Black BBKPowd. (GAF) (59850), Indocarbon CLGS Conc. CF (GAF) (53295), KatigenDeep Black NND Hi Conc. CF (GAF) (15711), Rapidogen Black 3 G (GAF)(Azoic Blk. 4); Sulphone Cyanine Black BA-CF (GAF) (26370), ZambeziBlack VD Ex Conc. (GAF) (30015); Rubanox Red CP-1495 (TheSherwin-Williams Company, Cleveland, Ohio) (15630); Raven 11 (ColumbianCarbon Company, Atlanta, Ga.), (carbon black aggregates with a particlesize of about 25 μm), Statex B-12 (Columbian Carbon Co.) (a furnaceblack of 33 μm average particle size), and chrome green.

[0069] Particles may also include laked, or dyed, pigments. Lakedpigments are particles that have a dye precipitated on them or which arestained. Lakes are metal salts of readily soluble anionic dyes. Theseare dyes of azo, triphenylmethane or anthraquinone structure containingone or more sulphonic or carboxylic acid groupings. They are usuallyprecipitated by a calcium, barium or aluminum salt onto a substrate.Typical examples are peacock blue lake (CI Pigment Blue 24) and Persianorange (lake of CI Acid Orange 7), Black M Toner (GAF) (a mixture ofcarbon black and black dye precipitated on a lake).

[0070] A dark particle of the dyed type may be constructed from anylight absorbing material, such as carbon black, or inorganic blackmaterials. The dark material may also be selectively absorbing. Forexample, a dark green pigment may be used. Black particles may also beformed by staining latices with metal oxides, such latex copolymersconsisting of any of butadiene, styrene, isoprene, methacrylic acid,methyl methacrylate, acrylonitrile, vinyl chloride, acrylic acid, sodiumstyrene sulfonate, vinyl acetate, chlorostyrene,dimethylaminopropylmethacrylamide, isocyanoethyl methacrylate andN-(isobutoxymethacrylamide), and optionally including conjugated dienecompounds such as diacrylate, triacrylate, dimethylacrylate andtrimethacrylate. Black particles may also be formed by a dispersionpolymerization technique.

[0071] In the systems containing pigments and polymers, the pigments andpolymers may form multiple domains within the electrophoretic particle,or be aggregates of smaller pigment/polymer combined particles.Alternatively, a central pigment core may be surrounded by a polymershell. The pigment, polymer, or both can contain a dye. The opticalpurpose of the particle may be to scatter light, absorb light, or both.Useful sizes may range from 1 nm up to about 100 μm, as long as theparticles are smaller than the bounding capsule. In a preferredembodiment, the density of the electrophoretic particle may besubstantially matched to that of the suspending (i. e., electrophoretic)fluid. As defined herein, a suspending fluid has a density that is“substantially matched” to the density of the particle if the differencein their respective densities is between about zero and about two g/ml.This difference is preferably between about zero and about 0.5 g/ml.

[0072] Useful polymers for the particles include, but are not limitedto: polystyrene, polyethylene, polypropylene, phenolic resins, Du PontElvax resins (ethylene-vinyl acetate copolymers), polyesters,polyacrylates, polymethacrylates, ethylene acrylic acid or methacrylicacid copolymers (Nucrel Resins—DuPont, Primacor Resins—Dow Chemical),acrylic copolymers and terpolymers (Elvacite Resins, DuPont) and PMMA.Useful materials for homopolymer/pigment phase separation in high shearmelt include, but are not limited to, polyethylene, polypropylene,polymethylmethacrylate, polyisobutylmethacrylate, polystyrene,polybutadiene, polyisoprene, polyisobutylene, polylauryl methacrylate,polystearyl methacrylate, polyisobornyl methacrylate, poly-t-butylmethacrylate, polyethyl methacrylate, polymethyl acrylate, polyethylacrylate, polyacrylonitrile, and copolymers of two or more of thesematerials. Some useful pigment/polymer complexes that are commerciallyavailable include, but are not limited to, Process Magenta PM 1776(Magruder Color Company, Inc., Elizabeth, N.J.), Methyl Violet PMAVM6223 (Magruder Color Company, Inc., Elizabeth, N.J.), and Naphthol FGRRF6257 (Magruder Color Company, Inc., Elizabeth, N.J.).

[0073] The pigment-polymer composite may be formed by a physicalprocess, (e.g., attrition or ball milling), a chemical process (e.g.,microencapsulation or dispersion polymerization), or any other processknown in the art of particle production. From the following non-limitingexamples, it may be seen that the processes and materials for both thefabrication of particles and the charging thereof are generally derivedfrom the art of liquid toner, or liquid immersion development. Thus anyof the known processes from liquid development are particularly, but notexclusively, relevant.

[0074] New and useful electrophoretic particles may still be discovered,but a number of particles already known to those skilled in the art ofelectrophoretic displays and liquid toners can also prove useful. Ingeneral, the polymer requirements for liquid toners and encapsulatedelectrophoretic inks are similar, in that the pigment or dye must beeasily incorporated therein, either by a physical, chemical, orphysicochemical process, may aid in the colloidal stability, and maycontain charging sites or may be able to incorporate materials whichcontain charging sites. One general requirement from the liquid tonerindustry that is not shared by encapsulated electrophoretic inks is thatthe toner must be capable of “fixing” the image, i.e., heat fusingtogether to create a uniform film after the deposition of the tonerparticles.

[0075] Typical manufacturing techniques for particles are drawn from theliquid toner and other arts and include ball milling, attrition, jetmilling, etc. The process will be illustrated for the case of apigmented polymeric particle. In such a case the pigment is compoundedin the polymer, usually in some kind of high shear mechanism such as ascrew extruder. The composite material is then (wet or dry) ground to astarting size of around 10 μm. It is then dispersed in a carrier liquid,for example ISOPAR® (Exxon, Houston, Tex.), optionally with some chargecontrol agent(s), and milled under high shear for several hours down toa final particle size and/or size distribution.

[0076] Another manufacturing technique for particles drawn from theliquid toner field is to add the polymer, pigment, and suspending fluidto a media mill. The mill is started and simultaneously heated totemperature at which the polymer swells substantially with the solvent.This temperature is typically near 100° C. In this state, the pigment iseasily encapsulated into the swollen polymer. After a suitable time,typically a few hours, the mill is gradually cooled back to ambienttemperature while stirring. The milling may be continued for some timeto achieve a small enough particle size, typically a few micrometers indiameter. The charging agents may be added at this time. Optionally,more suspending fluid may be added.

[0077] Chemical processes such as dispersion polymerization, mini- ormicro-emulsion polymerization, suspension polymerization precipitation,phase separation, solvent evaporation, in situ polymerization, seededemulsion polymerization, or any process which falls under the generalcategory of microencapsulation may be used. A typical process of thistype is a phase separation process wherein a dissolved polymericmaterial is precipitated out of solution onto a dispersed pigmentsurface through solvent dilution, evaporation, or a thermal change.Other processes include chemical means for staining polymeric latices,for example with metal oxides or dyes.

[0078] B. Suspending Fluid

[0079] The suspending fluid containing the particles can be chosen basedon properties such as density, refractive index, and solubility. Apreferred suspending fluid has a low dielectric constant (about 2), highvolume resistivity (about 10Λ15 ohm-cm), low viscosity (less than 5cst), low toxicity and environmental impact, low water solubility (lessthan 10 ppm), high specific gravity (greater than 1.5), a high boilingpoint (greater than 90° C.), and a low refractive index (less than 1.2).

[0080] The choice of suspending fluid may be based on concerns ofchemical inertness, density matching to the electrophoretic particle, orchemical compatibility with both the electrophoretic particle andbounding capsule. The viscosity of the fluid should be low when you wantthe particles to move. The refractive index of the suspending fluid mayalso be substantially matched to that of the particles. As used herein,the refractive index of a suspending fluid “is substantially matched” tothat of a particle if the difference between their respective refractiveindices is between about zero and about 0.3, and is preferably betweenabout 0.05 and about 0.2.

[0081] Additionally, the fluid may be chosen to be a poor solvent forsome polymers, which is advantageous for use in the fabrication ofmicroparticles because it increases the range of polymeric materialsuseful in fabricating particles of polymers and pigments. Organicsolvents, such as halogenated organic solvents, saturated linear orbranched hydrocarbons, silicone oils, and low molecular weighthalogen-containing polymers are some useful suspending fluids. Thesuspending fluid may comprise a single fluid. The fluid will, however,often be a blend of more than one fluid in order to tune its chemicaland physical properties. Furthermore, the fluid may contain surfacemodifiers to modify the surface energy or charge of the electrophoreticparticle or bounding capsule. Reactants or solvents for themicroencapsulation process (oil soluble monomers, for example) can alsobe contained in the suspending fluid. Charge control agents can also beadded to the suspending fluid.

[0082] Useful organic solvents include, but are not limited to,epoxides, such as, for example, decane epoxide and dodecane epoxide;vinyl ethers, such as, for example, cyclohexyl vinyl ether and Decave®(International Flavors & Fragrances, Inc., New York, N.Y.); and aromatichydrocarbons, such as, for example, toluene and naphthalene. Usefulhalogenated organic solvents include, but are not limited to,tetrafluorodibromoethylene, tetrachloroethylene,trifluorochloroethylene, 1,2,4-trichlorobenzene, carbon tetrachloride.These materials have high densities. Useful hydrocarbons include, butare not limited to, dodecane, tetradecane, the aliphatic hydrocarbons inthe Isopar® series (Exxon, Houston, Tex.), Norpar® ( series of normalparaffinic liquids), Shell-Sol® (Shell, Houston, Tex.), and Sol-Trol®(Shell), naphtha, and other petroleum solvents. These materials usuallyhave low densities. Useful examples of silicone oils include, but arenot limited to, octamethyl cyclosiloxane and higher molecular weightcyclic siloxanes, poly (methyl phenyl siloxane), hexamethyldisiloxane,and polydimethylsiloxane. These materials usually have low densities.Useful low molecular weight halogen-containing polymers include, but arenot limited to, poly(chlorotrifluoroethylene) polymer (Halogenatedhydrocarbon Inc., River Edge, N.J.), Galden® (a perfluorinated etherfrom Ausimont, Morristown, N.J.), or Krytox® from DuPont (Wilmington,Del.). In a preferred embodiment, the suspending fluid is apoly(chlorotrifluoroethylene) polymer. In a particularly preferredembodiment, this polymer has a degree of polymerization from about 2 toabout 10. Many of the above materials are available in a range ofviscosities, densities, and boiling points.

[0083] The fluid must be capable of being formed into small dropletsprior to a capsule being formed. Processes for forming small dropletsinclude flow-through jets, membranes, nozzles, or orifices, as well asshear-based emulsifying schemes. The formation of small drops may beassisted by electrical or sonic fields. Surfactants and polymers can beused to aid in the stabilization and emulsification of the droplets inthe case of an emulsion type encapsulation. A preferred surfactant foruse in displays of the invention is sodium dodecylsulfate.

[0084] It can be advantageous in some displays for the suspending fluidto contain an optically absorbing dye. This dye must be soluble in thefluid, but will generally be insoluble in the other components of thecapsule. There is much flexibility in the choice of dye material. Thedye can be a pure compound, or blends of dyes to achieve a particularcolor, including black. The dyes can be fluorescent, which would producea display in which the fluorescence properties depend on the position ofthe particles. The dyes can be photoactive, changing to another color orbecoming colorless upon irradiation with either visible or ultravioletlight, providing another means for obtaining an optical response. Dyescould also be polymerizable, forming a solid absorbing polymer insidethe bounding shell.

[0085] There are many dyes that can be chosen for use in encapsulatedelectrophoretic display. Properties important here include lightfastness, solubility in the suspending liquid, color, and cost. Theseare generally from the class of azo, anthraquinone, and triphenylmethanetype dyes and may be chemically modified so as to increase thesolubility in the oil phase and reduce the adsorption by the particlesurface.

[0086] A number of dyes already known to those skilled in the art ofelectrophoretic displays will prove useful. Useful azo dyes include, butare not limited to: the Oil Red dyes, and the Sudan Red and Sudan Blackseries of dyes. Useful anthraquinone dyes include, but are not limitedto: the Oil Blue dyes, and the Macrolex Blue series of dyes. Usefultriphenylmethane dyes include, but are not limited to, Michler's hydrol,Malachite Green, Crystal Violet, and Auramine O.

[0087] C. Charge Control Agents and Particle Stabilizers

[0088] Charge control agents are used to provide good electrophoreticmobility to the electrophoretic particles. Stabilizers are used toprevent agglomeration of the electrophoretic particles, as well asprevent the electrophoretic particles from irreversibly depositing ontothe capsule wall. Either component can be constructed from materialsacross a wide range of molecular weights (low molecular weight,oligomeric, or polymeric), and may be pure or a mixture. In particular,suitable charge control agents are generally adapted from the liquidtoner art. The charge control agent used to modify and/or stabilize theparticle surface charge is applied as generally known in the arts ofliquid toners, electrophoretic displays, non-aqueous paint dispersions,and engine-oil additives. In all of these arts, charging species may beadded to non-aqueous media in order to increase electrophoretic mobilityor increase electrostatic stabilization. The materials can improvesteric stabilization as well. Different theories of charging arepostulated, including selective ion adsorption, proton transfer, andcontact electrification.

[0089] An optional charge control agent or charge director may be used.These constituents typically consist of low molecular weightsurfactants, polymeric agents, or blends of one or more components andserve to stabilize or otherwise modify the sign and/or magnitude of thecharge on the electrophoretic particles. The charging properties of thepigment itself may be accounted for by taking into account the acidic orbasic surface properties of the pigment, or the charging sites may takeplace on the carrier resin surface (if present), or a combination of thetwo. Additional pigment properties which may be relevant are theparticle size distribution, the chemical composition, and thelightfastness. The charge control agent used to modify and/or stabilizethe particle surface charge is applied as generally known in the arts ofliquid toners, electrophoretic displays, non-aqueous paint dispersions,and engine-oil additives. In all of these arts, charging species may beadded to non-aqueous media in order to increase electrophoretic mobilityor increase electrostatic stabilization. The materials can improvesteric stabilization as well. Different theories of charging arepostulated, including selective ion adsorption, proton transfer, andcontact electrification.

[0090] Charge adjuvants may also be added. These materials increase theeffectiveness of the charge control agents or charge directors. Thecharge adjuvant may be a polyhydroxy compound or an aminoalcoholcompound, which are preferably soluble in the suspending fluid in anamount of at least 2% by weight. Examples of polyhydroxy compounds whichcontain at least two hydroxyl groups include, but are not limited to,ethylene glycol, 2,4,7,9-tetramethyl-decyne-4,7-diol, poly(propyleneglycol), pentaethylene glycol, tripropylene glycol, triethylene glycol,glycerol, pentaerythritol, glycerol tris(12-hydroxystearate), propyleneglycol monohydroxystearate, and ethylene glycol monohydroxystearate.Examples of aminoalcohol compounds which contain at least one alcoholfunction and one amine function in the same molecule include, but arenot limited to, triisopropanolamine, triethanolamine, ethanolamine,3-amino-1-propanol, o-aminophenol,5-amino-1-pentanol, andtetrakis(2-hydroxyethyl)ethylene-diamine. The charge adjuvant ispreferably present in the suspending fluid in an amount of about 1 toabout 100 mg/g of the particle mass, and more preferably about 50 toabout 200 mg/g.

[0091] The surface of the particle may also be chemically modified toaid dispersion, to improve surface charge, and to improve the stabilityof the dispersion, for example. Surface modifiers include organicsiloxanes, organohalogen silanes and other functional silane couplingagents (Dow Corning® Z-6070, Z-6124, and 3 additive, Midland, Mich.);organic titanates and zirconates (Tyzor® TOT, TBT, and TE Series,DuPont, Wilmington, Del.); hydrophobing agents, such as long chain (C12to C50) alkyl and alkyl benzene sulphonic acids, fatty amines ordiamines and their salts or quaternary derivatives; and amphipathicpolymers which can be covalently bonded to the particle surface.

[0092] In general, it is believed that charging results as an acid-basereaction between some moiety present in the continuous phase and theparticle surface. Thus useful materials are those which are capable ofparticipating in such a reaction, or any other charging reaction asknown in the art.

[0093] Different non-limiting classes of charge control agents which areuseful include organic sulfates or sulfonates, metal soaps, block orcomb copolymers, organic amides, organic zwitterions, and organicphosphates and phosphonates. Useful organic sulfates and sulfonatesinclude, but are not limited to, sodium bis(2-ethyl hexyl)sulfosuccinate, calcium dodecyl benzene sulfonate, calcium petroleumsulfonate, neutral or basic barium dinonylnaphthalene sulfonate, neutralor basic calcium dinonylnaphthalene sulfonate, dodecylbenzenesulfonicacid sodium salt, and ammonium lauryl sulphate. Useful metal soapsinclude, but are not limited to, basic or neutral barium petronate,calcium petronate, Co—, Ca—, Cu—, Mn—, Ni—, Zn—, and Fe— salts ofnaphthenic acid, Ba—, Al—, Zn—, Cu—, Pb—, and Fe— salts of stearic acid,divalent and trivalent metal carboxylates, such as aluminum tristearate,aluminum octanoate, lithium heptanoate, iron stearate, iron distearate,barium stearate, chromium stearate, magnesium octanoate, calciumstearate, iron naphthenate, and zinc naphthenate, Mn— and Zn—heptanoate, and Ba—, Al—, Co—, Mn—, and Zn— octanoate. Useful block orcomb copolymers include, but are not limited to, AB diblock copolymersof (A) polymers of 2-(N,N)-dimethylaminoethyl methacrylate quaternizedwith methyl-p-toluenesulfonate and (B) poly(2-ethylhexyl methacrylate),and comb graft copolymers with oil soluble tails of poly(12-hydroxystearic acid) and having a molecular weight of about 1800,pendant on an oil-soluble anchor group of poly (methylmethacrylate-methacrylic acid). Useful organic amides include, but arenot limited to, polyisobutylene succinimides such as OLOA 1200 or 3700,and N-vinyl pyrrolidone polymers. Useful organic zwitterions include,but are not limited to, lecithin. Useful organic phosphates andphosphonates include, but are not limited to, the sodium salts ofphosphated mono- and di-glycerides with saturated and unsaturated acidsubstituents.

[0094] Particle dispersion stabilizers may be added to prevent particleflocculation or attachment to the capsule walls. For the typical highresistivity liquids used as suspending fluids in electrophoreticdisplays, nonaqueous surfactants may be used. These include, but are notlimited to, glycol ethers, acetylenic glycols, alkanolamides, sorbitolderivatives, alkyl amines, quaternary amines, imidazolines, dialkyloxides, and sulfosuccinates.

[0095] D. Encapsulation

[0096] There is a long and rich history to encapsulation, with numerousprocesses and polymers having proven useful in creating capsules.Encapsulation of the internal phase may be accomplished in a number ofdifferent ways. Numerous suitable procedures for microencapsulation aredetailed in both Microencapsulation, Processes and Applications, (I. E.Vandegaer, ed.), Plenum Press, New York, N.Y. (1974) and Gutcho,Microcapsules and Microencapsulation Techniques, Nuyes Data Corp., ParkRidge, N.J. (1976). The processes fall into several general categories,all of which can be applied to the present invention: interfacialpolymerization, in situ polymerization, physical processes, such ascoextrusion and other phase separation processes, in-liquid curing, andsimple/complex coacervation.

[0097] Numerous materials and processes should prove useful informulating displays of the present invention. Useful materials forsimple coacervation processes include, but are not limited to, gelatin,polyvinyl alcohol, polyvinyl acetate, and cellulosic derivatives, suchas, for example, carboxymethylcellulose. Useful materials for complexcoacervation processes include, but are not limited to, gelatin, acacia,carageenan, carboxymethylcellulose, hydrolyzed styrene anhydridecopolymers, agar, alginate, casein, albumin, methyl vinyl etherco-maleic anhydride, and cellulose phthalate. Useful materials for phaseseparation processes include, but are not limited to, polystyrene, PMMA,poly(ethyl methacrylate), poly(butyl methacrylate), ethyl cellulose,poly(vinyl pyridine), and polyacrylonitrile. Useful materials for insitu polymerization processes include, but are not limited to,polyhydroxyamides, with aldehydes, melamine, or urea and formaldehyde;water-soluble oligomers of the condensate of melamine, or urea andformaldehyde; and vinyl monomers, such as, for example, styrene, MMA andacrylonitrile. Finally, useful materials for interfacial polymerizationprocesses include, but are not limited to, diacyl chlorides, such as,for example, sebacoyl, adipoyl, and di- or poly-amines or alcohols, andisocyanates. Useful emulsion polymerization materials may include, butare not limited to, styrene, vinyl acetate, acrylic acid, butylacrylate, t-butyl acrylate, methyl methacrylate, and butyl methacrylate.

[0098] Capsules produced may be dispersed into a curable carrier,resulting in an ink which may be printed or coated on large andarbitrarily shaped or curved surfaces using conventional printing andcoating techniques.

[0099] In the context of the present invention, one skilled in the artwill select an encapsulation procedure and wall material based on thedesired capsule properties. These properties include the distribution ofcapsule radii; electrical, mechanical, diffusion, and optical propertiesof the capsule wall; and chemical compatibility with the internal phaseof the capsule.

[0100] The capsule wall generally has a high electrical resistivity.Although it is possible to use walls with relatively low resistivities,this may limit performance in requiring relatively higher addressingvoltages. The capsule wall should also be mechanically strong (althoughif the finished capsule powder is to be dispersed in a curable polymericbinder for coating, mechanical strength is not as critical). The capsulewall should generally not be porous. If, however, it is desired to usean encapsulation procedure that produces porous capsules, these can beovercoated in a post-processing step (i.e., a second encapsulation).Moreover, if the capsules are to be dispersed in a curable binder, thebinder will serve to close the pores. The capsule walls should beoptically clear. The wall material may, however, be chosen to match therefractive index of the internal phase of the capsule (i.e., thesuspending fluid) or a binder in which the capsules are to be dispersed.For some applications (e.g., interposition between two fixedelectrodes), monodispersed capsule radii are desirable.

[0101] An encapsulation procedure involves a polymerization between ureaand formaldehyde in an aqueous phase of an oil/water emulsion in thepresence of a negatively charged, carboxyl-substituted, linearhydrocarbon polyelectrolyte material. The resulting capsule wall is aurea/formaldehyde copolymer, which discretely encloses the internalphase. The capsule is clear, mechanically strong, and has goodresistivity properties.

[0102] The related technique of in situ polymerization utilizes anoil/water emulsion, which is formed by dispersing the electrophoreticcomposition (i.e., the dielectric liquid containing a suspension of thepigment particles) in an aqueous environment. The monomers polymerize toform a polymer with higher affinity for the internal phase than for theaqueous phase, thus condensing around the emulsified oily droplets. Inone especially useful in situ polymerization processes, urea andformaldehyde condense in the presence of poly(acrylic acid) (See, e.g.,U.S. Pat. No. 4,001,140). In other useful process, any of a variety ofcross-linking agents borne in aqueous solution is deposited aroundmicroscopic oil droplets. Such cross-linking agents include aldehydes,especially formaldehyde, glyoxal, or glutaraldehyde; alum; zirconiumsalts; and poly isocyanates. The entire disclosures of the U.S. Pat.Nos. 4,001,140 and 4,273,672 patents are hereby incorporated byreference herein.

[0103] The coacervation approach also utilizes an oil/water emulsion.One or more colloids are coacervated (i.e., agglomerated) out of theaqueous phase and deposited as shells around the oily droplets throughcontrol of temperature, pH and/or relative concentrations, therebycreating the microcapsule. Materials suitable for coacervation includegelatins and gum arabic.

[0104] The interfacial polymerization approach relies on the presence ofan oil-soluble monomer in the electrophoretic composition, which onceagain is present as an emulsion in an aqueous phase. The monomers in theminute hydrophobic droplets react with a monomer introduced into theaqueous phase, polymerizing at the interface between the droplets andthe surrounding aqueous medium and forming shells around the droplets.Although the resulting walls are relatively thin and may be permeable,this process does not require the elevated temperatures characteristicof some other processes, and therefore affords greater flexibility interms of choosing the dielectric liquid.

[0105] Coating aids can be used to improve the uniformity and quality ofthe coated or printed electrophoretic ink material. Wetting agents aretypically added to adjust the interfacial tension at thecoating/substrate interface and to adjust the liquid/air surfacetension. Wetting agents include, but are not limited to, anionic andcationic surfactants, and nonionic species, such as silicone orfluoropolymer based materials. Dispersing agents may be used to modifythe interfacial tension between the capsules and binder, providingcontrol over flocculation and particle settling.

[0106] Surface tension modifiers can be added to adjust the air/inkinterfacial tension. Polysiloxanes are typically used in such anapplication to improve surface leveling while minimizing other defectswithin the coating. Surface tension modifiers include, but are notlimited to, fluorinated surfactants, such as, for example, the Zonyl®series from DuPont (Wilmington, Del.), the Fluorod® series from 3M (St.Paul, Minn.), and the fluoroakyl series from Autochem (Glen Rock, N.J.);siloxanes, such as, for example, Silwet® from Union Carbide (Danbury,Conn.); and polyethoxy and polypropoxy alcohols. Antifoams, such assilicone and silicone-free polymeric materials, may be added to enhancethe movement of air from within the ink to the surface and to facilitatethe rupture of bubbles at the coating surface. Other useful antifoamsinclude, but are not limited to, glyceryl esters, polyhydric alcohols,compounded antifoams, such as oil solutions of alkyl benzenes, naturalfats, fatty acids, and metallic soaps, and silicone antifoaming agentsmade from the combination of dimethyl siloxane polymers and silica.Stabilizers such as uv-absorbers and antioxidants may also be added toimprove the lifetime of the ink.

[0107] Other additives to control properties like coating viscosity andfoaming can also be used in the coating fluid. Stabilizers(UV-absorbers, antioxidants) and other additives which could proveuseful in practical materials.

[0108] E. Binder Material

[0109] The binder is used as a non-conducting, adhesive mediumsupporting and protecting the capsules, as well as binding the electrodematerials to the capsule dispersion. Binders are available in many formsand chemical types. Among these are water-soluble polymers, water-bornepolymers, oil-soluble polymers, thermoset and thermoplastic polymers,and radiation-cured polymers.

[0110] Among the water-soluble polymers are the various polysaccharides,the polyvinyl alcohols, N-methylpyrrolidone, N-vinylpyrrolidone, thevarious Carbowax® species (Union Carbide, Danbury, Conn.), andpoly(2-hydroxyethyl acrylate).

[0111] The water-dispersed or water-borne systems are generally latexcompositions, typified by the Neorez® and Neocryl® resins (ZenecaResins, Wilmington, Mass.), Acrysol® (Rohm and Haas, Philadelphia, Pa.),Bayhydrol® (Bayer, Pittsburgh, Pa.), and the Cytec Industries (WestPaterson, N.J.) HP line. These are generally latices of polyurethanes,occasionally compounded with one or more of the acrylics, polyesters,polycarbonates or silicones, each lending the final cured resin in aspecific set of properties defined by glass transition temperature,degree of “tack,” softness, clarity, flexibility, water permeability andsolvent resistance, elongation modulus and tensile strength,thermoplastic flow, and solids level. Some water-borne systems can bemixed with reactive monomers and catalyzed to form more complex resins.Some can be further cross-linked by the use of a crosslinking reagent,such as an aziridine, for example, which reacts with carboxyl groups.

[0112] A typical application of a water-borne resin and aqueous capsulesfollows. A volume of particles is centrifuged at low speed to separateexcess water. After a given centrifugation process, for example 10minutes at 60×G, the capsules are found at the bottom of the centrifugetube, while the water portion is at the top. The water portion iscarefully removed (by decanting or pipetting). The mass of the remainingcapsules is measured, and a mass of resin is added such that the mass ofresin is between one eighth and one tenth of the weight of the capsules.This mixture is gently mixed on an oscillating mixer for approximatelyone half hour. After about one half hour, the mixture is ready to becoated onto the appropriate substrate.

[0113] The thermoset systems are exemplified by the family of epoxies.These binary systems can vary greatly in viscosity, and the reactivityof the pair determines the “pot life” of the mixture. If the pot life islong enough to allow a coating operation, capsules may be coated in anordered arrangement in a coating process prior to the resin curing andhardening.

[0114] Thermoplastic polymers, which are often polyesters, are molten athigh temperatures. A typical application of this type of product ishot-melt glue. A dispersion of heat-resistant capsules could be coatedin such a medium. The solidification process begins during cooling, andthe final hardness, clarity and flexibility are affected by thebranching and molecular weight of the polymer.

[0115] Oil or solvent-soluble polymers are often similar in compositionto the water-borne system, with the obvious exception of the wateritself. The latitude in formulation for solvent systems is enormous,limited only by solvent choices and polymer solubility. Of considerableconcern in solvent-based systems is the viability of the capsuleitself—the integrity of the capsule wall cannot be compromised in anyway by the solvent.

[0116] Radiation cure resins are generally found among the solvent-basedsystems. Capsules may be dispersed in such a medium and coated, and theresin may then be cured by a timed exposure to a threshold level ofultraviolet radiation, either long or short wavelength. As in all casesof curing polymer resins, final properties are determined by thebranching and molecular weights of the monomers, oligomers andcrosslinkers.

[0117] A number of “water-reducible” monomers and oligomers are,however, marketed. In the strictest sense, they are not water soluble,but water is an acceptable diluent at low concentrations and can bedispersed relatively easily in the mixture. Under these circumstances,water is used to reduce the viscosity (initially from thousands tohundreds of thousands centipoise). Water-based capsules, such as thosemade from a protein or polysaccharide material, for example, could bedispersed in such a medium and coated, provided the viscosity could besufficiently lowered. Curing in such systems is generally by ultravioletradiation.

[0118] While the invention has been particularly shown and describedwith reference to specific preferred embodiments, it should beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the spirit and scopeof the invention as defined by the appended claims.

What is claimed is:
 1. A mounted electrophoretic display assembly,comprising: a flexible substrate; an electrical connection formed onsaid flexible substrate and having first and second contact pads spacedfrom one another; an electrophoretic display element in electricalcommunication with said first contact pad; and a control circuit mountedon said flexible substrate and in electrical communication with saidsecond contact pad.
 2. The display assembly of claim 1, wherein saidcontrol circuit is connected to said second contact pad with a curable,electrically conductive thermoset.
 3. The display assembly of claim 1,wherein said control circuit is connected to said second contact padwith an electrically conductive ink.
 4. The display assembly of claim 1,wherein said control circuit is connected to said second contact padwith an electrically conductive paint.
 5. The display assembly of claim1, wherein said control circuit is connected to said second contact padby being removably mounted in a control circuit carrier that is inelectrical communication with said second contact pad.
 6. The displayassembly of claim 1 wherein said control circuit comprises anelectrophoretic display driver chip.
 7. A method of manufacturing anelectrophoretic display assembly, comprising the steps of: providing aflexible substrate; forming upon said substrate an electrical connectionhaving a first contact pad and a second contact pad spaced from oneanother; mounting upon said substrate a control circuit in electricalcommunication with said second contact pad; and providing anelectrophoretic display element in electrical communication with saidfirst contact pad.
 8. The method of claim 7, wherein the step of formingupon said substrate an electrical connection comprises a printingprocess.
 9. The method of claim 7, wherein the step of providing anelectrophoretic display element comprises a printing process.
 10. Amethod of manufacturing an electrophoretic display assembly, comprisingthe steps of: providing a first flexible substrate; forming upon saidfirst flexible substrate an electrical connection having a first contactpad and a second contact pad separated from each other; mounting on saidfirst flexible substrate a control circuit in electrical communicationwith said second contact pad; providing a second flexible substrate;disposing upon said second flexible substrate an electrophoretic displayelement; and disposing said first flexible substrate adjacent saidsecond flexible substrate so that said first contact pad addresses saidelectrophoretic display element.
 11. The method of claim 10, wherein thestep of disposing upon said second flexible substrate an electrophoreticdisplay element comprises a printing process.
 12. The method of claim10, wherein the step of disposing said first flexible substrate adjacentsaid second flexible substrate further comprises a laminating process.